1. Field of the Invention
The present invention relates to an exhaust valve for an internal combustion engine, particularly a two-stroke crosshead engine, comprising a movable spindle with a valve disc which at its upper surface has a annular seat area of a material different from the base material of the valve disc, which seat area abuts a corresponding seat area on a stationary valve member in the closed position of the valve.
2. Description of Related Art
The development of exhaust valves for internal combustion engines has aimed for many years at extending the life and reliability of the valves. This has been done so far by manufacturing the valve spindles with a hot-corrosion-resistant material on the lower disc surface and a hard material in the seat area.
The seat area is particularly crucial for the reliability of the exhaust valve, as the valve has to close tightly to function correctly. It is well-known that the ability of the seat area to close tightly can be reduced by corrosion in a local area by a so-called burn through, where across the annular sealing surface a channel-shaped gutter emerges, through which hot gas flows when the valve is closed. Under unfortunate circumstances, this failure condition can arise and develop into a rejectable valve during less than 80 hours' operation, which means that often it is not possible to discover the beginning failure at the usual overhaul. Therefore, a burn through in the valve seat may cause unplanned shut-downs. If the engine is a propulsion engine in a ship, the condition may be initiated and develop into a failing valve during a single voyage between two ports, which may cause problems during the voyage and unintended expensive waiting time in port.
With a view to preventing burn throughs in the valve seat many different valve seat materials with ever increasing hardnesses have been developed over the years to make the seat wear-resistant by means of the hardness and reduce the formation of dent marks. The dent marks are a condition for development of a burn through as the dents may create a small leak through which hot gas flows. The hot gas can heat the material around the leak to a level of temperature where the gas with the aggressive components has a corrosive effect on the seat material so that the leak rapidly grows larger and the leakage flow of hot gas increases, which escalates the erosion. in addition to the hardness, seat materials have also developed towards a higher hot corrosion resistance to delay erosion after the occurrence of a small leak. The special requirements to the seat material and the deviant special requirements to material properties in other areas of the movable valve member necessitate a seat area of a material different from the base material of the valve disc, which also provides manufacturing advantages. A number of examples of known seat materials will be given below.
WO92/13179, for example, describes the use of the nickel-based alloy Alloy 50, the cobalt-based alloy Stellite 6 and a nickel-based alloy the most important alloy components of which are 20-24% Cr, 0.2-0.55% C and 4-7% Al. One object mentioned is that seat materials should be hard to reduce the formation of dent marks.
SE-B-422 388 describes a valve for an internal combustion engine having a base body made of a chromium-containing nickel alloy on which a chromium-containing cobalt alloy is deposited at a temperature exceeding 3000.degree. C., whereupon the body is exposed to mechanical treatment and aging at a temperature higher than the operating temperature. An object of this is to improve the corrosion resistance of the seat material and impart a high hardness to it.
DK-B-165125 teaches an exhaust valve for an internal combustion engine with a seat area of a corrosion-resistant facing alloy comprising 13-17% Cr, 2-6% Al, 0.1-8% Mo, 1.5-3.5% B, 0.5-3% Ti, 4-7% Co and a balance of Ni. High hardness of the seat material is desired.
U.S. Pat. No. 4,425,300 teaches a welded-on hardfacing alloy comprising 10-25% Cr, 3-15% Mo, 3-7% Si, 1-1.2% C, 1-30% Fe and a balance of Ni. The alloy is without porosity and has a hardness comparable with that of cobalt-based alloys.
EP-A-0529208 teaches a nickel and chromium containing hardfacing alloy for welding-on in the valve seat area in a car engine. The alloy contains 30-48% Ni, 1.5-15% W and/or 1.0-6.5% Mo and the balance is of at least 40% Cr. W and Mo have a solution-strengthening effect on the alloy. C can be added in amounts from 0.3 to 2.0% to increase the hardness by carbide formation, and B can be added in amounts from 0.1 to 1.5% to increase the hardness by chromium boride formation. Nb can be added in amounts from 1.0 to 4.0% for formation of hardness-increasing intermetallic compounds as well as carbides and borides.
EP-A-0 521 821 teaches a valve made of NIMONIC 80A or NIMONIC 81, which is provided with a layer of INCONEL 625 or of INCONEL 671 in the seat area to impart to the seat a higher corrosion resistance than the NIMONIC base body. The publication mentions for the alloy INCONEL 671 that it only has to be welded on, while for the alloy INCONEL 625 it mentions that after the welding it contains a dendritic carbide structure and that the seat area therefore has to be hot-worked to homogenise the carbide distribution in the structure to improve corrosion resistance.
The book `Diesel engine combustion chamber materials for heavy fuel operation` published in 1990 by The Institute of Marine Engineers, London, collects the experience gained for exhaust valve materials in a number of articles and provides recommendations as to how to design valves to achieve long life. Concerning valve seats the articles unanimously direct that the seat material has to have a high hardness and be of a material with a high resistance against hot corrosion. A number of different preferred materials for exhaust valves are described in Paper 7 of the book `The physical and mechanical properties of valve alloys and their use in component evaluation analyses`, including in its analysis of the mechanical properties of the materials a comparative table of the yield strength of the materials, seen to be below about 820 MPa.